If the fix does not work, please send the following to support@mcneel.com:

A screenshot of the error

Repair Utility log (all the text below RECOMMENDED ACTION)

The RHI installer log. Here is where you find it in V5, V6 or V7:

Rhino 5: `%appdata%\mcneel\rhinoceros\5.0\logs`

Rhino 6: `%appdata%\mcneel\rhinoceros\6.0\logs`

Rhino 7: `%appdata%\mcneel\rhinoceros\7.0\logs`

Old Rhino 5: attempting to install Grasshopper into Rhino V5 SR0 can generate this error. Update your Rhino 5 to the current service release to fix this problem.

Corrupt Download: a corrupt RHI file that cannot be installed. This often is the result of an incomplete download. Please delete the RHI file, clear your web browser's temporary files, and download again.

Poorly Authored RHI: if you are a developer, you may have not properly created the RHI file. Please review instructions for authoring RHI files.